Assessing the fate of emerging organic contaminants in a coupled surface water-groundwater system

For decades, wastewater containing significant amounts of emerging organic contaminants (EOCs) has been discharged into the aquatic environment. The introduction of EOCs into aquifers due to losing conditions of polluted rivers poses a serious risk for groundwater quality, which is particularly concerning in catchments with drinking water production facilities. Thus, understanding EOCs’ reactive transport processes in connected groundwater-surface water (GW-SW) systems are crucial for assessing their impact on water resources.

The present study investigates the GW-SW interaction at the Landgraben stream which is located within the Hessian Ried, Germany. Treated municipal and industrial wastewaters have been discharged into the Landgraben for decades, introducing a wide range of EOCs into the aquatic environment. To monitor the GW-SW interaction, we installed a complex monitoring system within and along a transect next to the Landgraben stream. The aims of our study are (i) to improve the understanding of the behaviour and fate of EOCs within the GW-SW interface and (ii) to link EOCs behaviour to the hydraulic responses of the seasonally varying stream. For this, we analysed 22 EOCs, as well as major ions, trace elements, dissolved organic carbon, rare earth elements and water stable isotopes over 16 months comprising two summers. Water samples were collected from the river, the hyporheic zone and the nearby groundwater at three different distances and at three depths at intervals ranging from biweekly to monthly.

Results show that the infiltration dynamics were strongly influenced by seasonal groundwater fluctuations, with influent conditions during summer and predominantly effluent conditions during winter. The second summer was characterized by a pronounced infiltration of river water into the aquifer driven by the preceding winter precipitation deficit.

The hydrochemical analysis showed a wide concentration range of EOCs in the river water samples. Of the compounds analysed, only Atenolol and Ciprofloxacin were not detected across the 29 sampling campaigns. Concentrations of the remaining 20 EOCs ranged from a few ng L⁻¹, for example Fluconazole (median = 52 ng L⁻¹), to several µg L⁻¹, such as Oxipurinol (median = 1957 ng L⁻¹). Overall, the detected EOCs cover a broad spectrum of chemical speciation and polarity, implying substantially different transport and attenuation behaviours along the SW-GW pathway. This variability is reflected in the sampled groundwater, where some compounds are no longer detected (eg., Sitagliptin, Venlafaxine), whereas others persist and even exhibit higher concentrations than those found in the stream during the sampling period (eg., Candesartan, Carbamazepine). Thus, this study highlights the significance of GW-SW interactions in the transport and attenuation of EOCs, providing insights into their differing fates within aquatic systems and how these relate to physicochemical properties.